I want to replace an old fuse box with a circuit breaker panel. Do I need a 200 AMP service? (considering putting in air conditioning in the future). The house has two apartments so I want to make sure there is adequate service. As I know absolutely nothing about this, what questions should I ask the electrician? Also outside service wiring has to be replaced. I'd appreciate any advice.

Ask an electrician that would like to bid on the project to do the service rating calculations. He should ask for the total square foot, and a list of all the major electric appliances, type of heat, size of future AC, etc.

Doing the calcs is the only sure method of determining the size service needed. No guessing here. Sizing a service is done using Article 220 of the National Electric Code.

Be sure any contractor that does the job is licensed , if applicable in your state, has the proper liability and worker comp insurances, and applies for permits and inspection if applicable.

The outside wiring on the building is owned and maintained by the owner, and will be sized according the calcs. If the utility line is aerial, the utility usually takes care of it, if underground, the you usually own and maintain these, depends on your local utility. You can check with your utility to see who owns what and what your responsibilities are.

I want to replace a 100amp fuse box with a circuit breaker box. What do I need to do?

Same thing TK told dvw2442.